Optical distributed antenna systems based on the combined use of radio frequency (RF) and optical signals are known, and used for example in radio-over-fiber systems. FIG. 1 shows schematically a known optical DAS 100 which includes a head-end (HE) unit 102 connected over a plurality N of point-to-point (P2P) optical fibers 104-1 . . . 104-N to N (N≧1) remote units (RUs) 106-1 . . . 106-N. Each RU may be connected to a passive DAS which includes a coax cable 112 and one or more antennas 114. Exemplarily in DAS 100, N=8. HE unit 102 includes one optical transmitter (exemplarily a diode laser) TX-0, N=8 receivers (typically photodiodes) RX-1 . . . RX-8 and N optical interfaces (ports) 108-1 . . . 108-8. Each remote unit includes RX and TX functionalities, provided in some cases by an electro-optical absorption modulator (EAM). Uplink (UL), the EAM modulates a RF signal received via the antenna into an optical signal which is transmitted to the head-end unit. Downlink (DL), the EAM converts an optical signal into a RF signal (i.e. acts as an optical detector). The UL and DL optical signals are transmitted over separate optical fibers, i.e. each EAM has two separate optical interfaces in addition to one RF port. In some applications, the EAM may have a multi-quantum well (MQW) structure, with its operation based on the quantum confined Stark effect (QCSE). A major disadvantage of such an EAM acting as an optical detector is its low efficiency, because the DL optical signal makes just one “pass” before being absorbed by the EAM structure.
The remote unit TX functionality may also be provided by a reflective optical transmitter (ROT), which joins or integrates an EAM having a reflective facet with a semiconductor optical amplifier (SOA). The combination is sometimes called SOA-EAM or REAM. However, such transmitters need a voltage input to bias the SOA in addition to a modulating electrical signal applied to the EAM. In other words, a SOA-EAM device has one optical interface, one RF port and one voltage source coupled thereto. Therefore, in known art, components in a RU which act as both receivers (detectors) of DL signals and transmitters of UL signals include always three ports or inputs/outputs.
Returning now to FIG. 1 and exemplarily, four wireless RF services (Bands 1, 2, 3 and 4) are combined and multiplexed in HE unit 102. The combined RF signal is converted into an optical signal with wavelength λ0 (hereinafter, “wavelength λ” is referring to simply as “λ”). The optical signal is split to optical interfaces 108-1 . . . 108-8 for DL transmission over a respective optical fiber to each remote unit. Each RU performs optical-to-RF conversion of the DL signal and outputs a RF signal to one or more antennas. Uplink, each RU receives a RF signal from an antenna and converts it into a λ1 optical signal which is transmitted to the HE unit. In known optical DAS, the source of the UL λ1 optical signal is either in the RU (i.e. the RU includes an optical transmitter) or remote from the RU, with λ1 input to the EAM and modulated thereby.
Communication networks are usually built in a hierarchical topology. Such topology is more scalable and flexible, enabling to design the network more efficiently. Consequently, deployment of hierarchical DAS topologies would be beneficial to an operator. However, the architecture of DAS 100 is “flat” in the sense that is does not allow an operator to design and deploy more efficient hierarchical DAS topologies. This arises from the use in DAS 100 of a P2P fiber between the HE and remote units, without any intermediate aggregation unit. The “flatness” problem may be solved by a hierarchical architecture shown in FIG. 2, in which an optical DAS 200 includes a C/DWDM (coarse/dense wavelength division multiplexer) or, equivalently, N WDM components positioned between a HE unit 202 and N remote units 206-1 . . . 206-8. Each RU is deployed with a different wavelength. The HE unit uses a λ0 for DL transmission, while each RU uses a different λN for UL transmission. Exemplarily, λ0 may be in the 1310-1330 nm wavelength range, while λN may be in the 1530-1560 nm wavelength range. Because the order of the different λN must be maintained in a C/DWDM component, the operator needs to administer these wavelengths. This is a major disadvantage. In addition, because each RU uses a different UL wavelength, the operator needs to maintain a stock of different RU transmitters.
Therefore, there is a need for and it would be advantageous to have simplified and efficient optical DAS architectures which overcome concurrently the “flatness” problem, the need to use a C/DWDM unit or multiple WDM units between head-end and remote units, and/or the need to maintain a stock of different RU transmitters. Such simplified and efficient optical DAS architectures will thereby reduce maintenance costs and increase product reliability and mean time between failures (MTBF). They will also reduce the need for different remote units and the need for special wavelength design.